1. Field of the Invention
The present invention relates to an optical scanning device which introduces light beams to a plurality of image forming portions in an image forming apparatus such as a laser beam printer or a digital copying machine, and an image forming apparatus.
2. Description of the Related Art
In general, an optical scanning device includes an optical box 31, as illustrated in FIG. 4. Inside of the optical box 31 are housed a light source unit 35 having a semiconductor laser or a collimator lens unified as a unit and a cylindrical lens 39 which converts a laser beam of a collimated optical flux illuminated by the light source unit 35 into a convergent beam. There is further provided a rotary polygon mirror 36 which deflects and scans the convergent light beam emitted from the cylindrical lens 39.
In addition, inside of the optical box 31 are housed image focusing lenses 37 which focus the deflected and scanned laser beam into an image and a returning mirror 38 which reflects a light beam emitted from the image focusing lens 37 and returns it. Moreover, there are provided four photosensitive drums (i.e., image carriers) 2 illuminated with the light beam reflected on the returning mirror 38.
The laser beam emitted from the light source unit 35 is linearly focused on a reflection surface of the rotary polygon mirror 36 by the cylindrical lens 39. The laser beam is reflected on the reflection surface of the rotary polygon mirror 36, and then, illuminates the electrically charged photosensitive drum 2 via the image focusing lens 37 and the returning mirror 38 in sequence.
At this time, the laser beam scans a surface to be scanned of the photosensitive drum 2 in main scanning direction, which is perpendicular to a direction of the rotation of the photosensitive drum 2, by a uniform motion of the rotary polygon mirror 36 and scans it in sub-scanning direction which is a direction of the rotation of the photosensitive drum 2, by the rotation of the photosensitive drum 2. In this manner, the surface to be scanned of the photosensitive drum 2 is exposed to the light beam, thereby forming an electrostatic latent image. Thereafter, the electrostatic latent image is developed to a visible image, which is then transferred and fixed to a sheet.
In the above-described optical scanning device, there has been known the following configuration. That is to say, four rotary polygon mirrors corresponding to the four photosensitive drums are disposed independently of each other, and they deflect and scan the light beam, to thus form an electrostatic latent image on the photosensitive drum (see Japanese Patent Laid-Open 2004-021138, hereinafter “Patent Literature 1”).
Otherwise, an optical scanning device illustrated in FIG. 5 is a type having a so-called 2-in-1 configuration which one rotary polygon mirror 41 deflects and scans a light beam with respect to two optical paths in order to save a space.
However, a plurality of rotary polygon mirrors 41a and 41b are arranged in series in either of a configuration disclosed in Patent Literature 1 and the 2-in-1 configuration, and therefore, a width in an arrangement direction of the rotary polygon mirrors becomes larger, thereby increasing the widthwise size of the image forming apparatus. Alternatively, an image forming apparatus can be reduced in width but needs twice or more space in a height direction in a configuration in which the optical scanning devices are superimposed in the height direction, like the configuration in Patent Literature 1.
In view of this, a light beam is emitted at four different angles in a sub-scanning direction by one rotary polygon mirror, and then, four photosensitive drums are independently illuminated via returning mirrors corresponding to optical paths, respectively (see Japanese Patent Laid-Open 2001-264655 hereinafter “Patent Literature 2”).
However, a distance from the center of a rotational axis to a reflection surface may be varied per surface according to an error of a machining accuracy of the rotary polygon mirror in an optical scanning device disclosed in Patent Literature 2. In this case, in this configuration in which the light beam is incident slantwise into one rotary polygon mirror in the sub-scanning direction, there arises a situation in which a dot position on the photosensitive drum is deviated per surface in the sub-scanning direction, thereby markedly degrading a quality of an image.
With this being the situation, it may be construed that an interference of a light beam in an arrangement direction of a plurality of rotary polygon mirrors is prevented by not inclining an angle of a light beam incident into a rotary polygon mirror but inclining a rotational axis of at least either one rotary polygon mirror. However, when the inclinations of the rotational axes of the rotary polygon mirrors are different from each other, deviations at scanning positions depend upon the rotary polygon mirrors, thereby growing concern about a quality of an image.